Ajax reached the Champions League knockout phase for the first time in 13 years but the Dutch club's 2-0 win at AEK Athens was marred by violent clashes both in and outside the Athens Olympic stadium on Tuesday.
Visiting supporters had to shield themselves from flames during clashes with Greek riot police prior to the start of the game after a petrol bomb and flares were reportedly thrown in the direction of the Ajax fans.
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Pictures showed Ajax fans clashing with Greek police and at least two supporters being left bloodied.
Eleven people were "hospitalized" with minor injuries, a police spokesman told AFP.
"AEK FC unreservedly and by all means condemns the incidents of violence that took place ... not only because they happened during a match for [the] UEFA Champions League, but also because they insult the long history of our club," said a statement on the Greek club's website.
"We worked very hard in every level in order to return to the most important European club competition; what happened during [the] AFC Ajax home clash does not represent the timeless football philosophy of our club and is completely against our principles and the values we honor and serve.
"The obvious thing is condemning what happened on Tuesday and the most important thing is taking all necessary action in order for such incidents not to take place ever again. We have proved that we know well how to correct our mistakes."
UEFA has opened disciplinary proceedings against both clubs, while Ajax is yet to comment.
Ajax defender Matthijs de Ligt tried to calm things down ahead of the Group E contest. Firecrackers then went off in the second half.
Disturbances in Greek stadiums are nothing new.
In March the country's Superleague was temporarily suspended when PAOK owner Ivan Savvidis entered the pitch with a gun in the late stages of his team's 0-0 draw against AEK. A PAOK goal had just been disallowed for offside.
He later received a three-year ban from football and PAOK was docked three points.
Three years ago the league was also temporarily suspended when fan trouble erupted in an Athens derby featuring the top two teams in Greece historically, Olympiakos and Panathinaikos.
Dusan Tadic starred for Ajax on the pitch Tuesday, opening the scoring in the 68th minute from the penalty spot after a handball -- which led to a second yellow card for AEK's Marko Livaja -- and adding the second four minutes later.
Progressing in Europe's top club competition was made more impressive because Ajax -- which won the last of its four titles in 1995 -- had to go through three rounds of qualifying and has long sold its best performers.
It still holds the most Dutch domestic league titles, although PSV leads Ajax by five points this season and has won in three of the past four seasons.
"Ajax deserve to start winning trophies," Tadic was quoted as saying by UEFA. "First we set as a goal that we would go to the Champions League -- we did that. Then we aimed to get to the next stage of the UCL, which we did as well.
"That's how you do it, setting goals and achieving them, in Europe and the Netherlands. We will try our best in all competitions and work hard for trophies."
Ajax will win Group E if it beats Bayern Munich on the final matchday next month.
Real Madrid also through
Real Madrid might be struggling domestically but Los Blancos know how to win trophies in the Champions League being the three-time defending champions.
Real Madrid and Tuesday's opponents Roma were already guaranteed passage into the round of 16 from Group G after CSKA lost to Plzen 2-1 in Moscow but the Spanish outfit still beat last season's semifinalists 2-0 in the Italian capital.
How things though could have been different had Cengiz Under not missed a golden chance for Roma in first-half injury time. A few yards from goal and with keeper Thibaut Courtois stranded, he ballooned over the bar. It was one of the misses of the season.
Gareth Bale proceeded to break the deadlock just moments after the interval -- notching his third Champions League goal of the season -- before Lucas Vazquez doubled the advantage in the 59th.
"We're playing against a side of champions and the difference can be seen when they score with their first chance," Roma manager Eusebio Di Francesco was quoted as saying by UEFA.
Real Madrid thus recovered from losing for the first time ever against Eibar last weekend -- a match where Courtois struggled -- in Santi Solari's first game in permanent charge.
The troubles continue however for Di Francesco. Roma sits a staggering 18 points behind Juventus in Serie A and entered Tuesday's game on the back of a shocking 1-0 loss to Udinese, which had lost 10 in a row to The Giallorossi in the league and had just fired its manager.
Half the lineup complete
Eight clubs now find themselves in the last 16: Barcelona -- the lone team through prior to the current matchday -- Bayern Munich, Ajax, Manchester City, Real Madrid, Roma, Juventus and Manchester United.
Manchester United still isn't firing on all cylinders, needing an outstanding David de Gea save and an injury time winner from Marouane Fellaini to beat Young Boys 1-0 at Old Trafford. United trails Juventus by two points in Group H after the 'Old Lady' of Italian football edged Valencia 1-0.
Cristiano Ronaldo assisted on Mario Mandzukic's winner in the 59th minute.
United's city rivals Manchester City, meanwhile, rescued a point when Sergio Aguero netted in the 83rd minute for a 2-2 draw at Lyon in Group F. Robert Lewandowski scored twice for Bayern in its 5-1 win over Benfica and in the process became the seventh player to register 50 Champions League goals overall.